Reed relay having low thermal emf

ABSTRACT

A reed relay switch consists of a pair of low reluctance, ferromagnetic, flattened reeds, which are hermetically sealed into a glass tube. The ferromagnetic material extends out of the glass tube and to a juncture with interconnecting conductors to external circuits. These junctures are usually of dissimilar metals and generate thermal EMF&#39;&#39;s. A thermally conductive electrical insulating body is provided and the connections are made at either side of this body so that the thermal EMF&#39;&#39;s produced by the thermally active junctions tend to cancel each other and reduce the thermal EMF across the reed relay.

United States Patent [451 Oct. 31, 1972 Campbell Y [54] REED RELAY HAVING LOW THERMAL EMF [72] Inventor: Bruce D. Campbell, Foster, R.l.

[73] Assignee: Cote-Coil Co., Inc.

[22] Filed: July 12, 1971 [21] Appl.No.: 161,781

[52] US. .Cl ..335/l54 [51] Int. CL... ..H01h 45/14 [58] Field of Sem'ch..... ..335/97, 151, 154, 163

[56] References Cited UNITED STATES PATENTS 3,488,760 l/l970 Julie ..335/l54' 3,005,069 10/1961 Sippach et al. ........335/l63 X Primary Examiner-Roy N. Envall, Jr. Attorney-Herbert B. Barlow et al.

[57] ABSTRACT A reed relay switch consists of a pair of low reluctance, ferromagnetic, flattened reeds, which are hermetically sealed into a glass tube. The ferromagnetic material-extends out of the glass tube and to a juncture with interconnecting conductors to external circuits. These junctures are usually of dissimilar metals and generate'thermal EMFs. A thermally conductive electrical insulating body is provided and the connections are made at either side of this body so 7 7 Claims, 8"Drawing Figures I REED RELAY HAVING LOW THERMAL EMF BACKGROUND OFTHE INVENTION A reed relay consists of two or more metal reeds which are enclosed in a hermetically sealed glass capsule. The reeds are made from nickel-iron, a magnetitaining the two junctions at the same temperature by attaching them both to opposite sides of a thin thermally conductive electrical insulator. When a high cally soft alloy, that retains only little magnetism. The

. held apart except in the presence of the magnetic field.

As mentioned before, the reeds are composed of a nickel-iron alloy,'one suitable alloy being sold under the tradename of Niron and extensions of this reed structure are brought out through the glass enclosure to form the ends of the reed switch. To make a connection to an external circuit, it is almost universal to utilize the material which is copper although oftentimes the material is tinned copper or .gold or silver plated copper, and in certain applications may even be silver or gold. The interconnection material generally being chosen to avoid any further thermoelectric junctions with the external circuitry. At the points where the interconnection material and the magnetic material make contact, junctions of dissimilar materials are made, and'these junctions generate thermal EMFs due to the thermoelectric effect. For example, typical reed relay switches produce thermal EMFs ranging from ,to 100 microvolts, and thermal differences of this magnitude cause serious problems when microvolt signals must be switched. One of the, problems has been the fact that the two junctionsare made on opposite ends of a switch capsule and, therefore, cannot be .maintained at the same temperature, temperature differences of 1 C. sometimes giving rise to thermal EMF differences of 50 microvolt magnitude. Accordingly, in order to maintain an offset across the switch of less than 0.5 microvolts, the two thermal, junctions must be less than 0.01C apart.

It is known, for example, that the temperature differentials are created in a number of ways, such as by heat dissipated in the actuating coil or a solenoid, different convective air currents, external heat generators in the circuitry in which the switch is used and possibly some other factors not specifically mentioned. In the past there has been at least one attempt to correct this deficiency in a reed relay, and this has been disclosed in the Becker et al US. Pat. No. 3,456,216. The attempt that was made here was to serially connect a pair of reed switches in such a fashion that the thermal juncdegree'of accuracy is desired, it is further possible to remove the junctions from all heat sources, that is, by physically separating the junctions from the solenoid drive coil and prevent that coil from contributing to a temperature differential. For example, the ferro magnetic alloy-copper junctions can be placed on top of the coil by extending .the ferro magnetic alloy leads a sufficient distance to place the junctions substantially mid- I way along the coil over the reed switch.

tions were made at the same end of the body, and in SUMMARY OF THE INVENTION It is, therefore, the main object of this invention to provide a technique by which practically any reed switch can be modified to produce a reed relay having a thermal EMF of less than lmicrovolt and yet maintain the package size as small as has been practical in the past. This object is achieved by diminishing the temperature difference between the two ferro magnetic alloy-copper junctions by the simple expedient of main- DESCRIPTION OFTHE DRAWINGS '1 FIG. 1 is a perspective view partly in section of a reed relay manufactured in accordance with the invention;

v FIG. 1A is a diagrammatic view of a modified configuration;

FIGflB is a diagrammatic view of another modified configuration;

FlG. 2 is a sectional view taken along lines 2-2 of FIG. 'lillustrating the thermally conductive electrical insulating body with the leads soldered thereto;

FIG/2A is a sectional view of a modification of the showing of FIG. 2;

e FIG. 2B is a perspective view of a different modifica- A tion of FIG. 2;

FIG. 3 is a sectional view of an encapsulated low thermal EMF switch which may be actuated by any form of magnetic field; and

I FIG. 4 is a view of a modified form'of switch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS surfaces are plated with a suitable metal topromote lower contact resistance and generally a chemically "inert gas is enclosed in the capsule which further contributes to the maintenance of good contact properties. The glass capsule is surrounded by a magnetic operating means in the form of a coil or solenoid which may be wound about a bobbin 15 which has a pair of spaced end walls 16, 17. If a bobbin is used it may be fonned of a plastic material such as nylon or any other suitable non magneticmaterial and in all forms define an elongated housing in which the switch 10 is received and suitably held in position as by elastic buffers or other devices indicated by the numerals 18. The coil 20 is normally wound with ve'ry'fine copper wire of a size,

voltage rating and a number of turns that will actuate the particular ree'd switch 10, which is mounted therein. It is preferable that the coil be wound evenly and symmetrically to provide even dissipation of heat throughout its surface.

The nickel-iron alloy reeds which extend outwardly from the sealed capsule 13 may be said to have end portions thereof which shall be designated 22, 23. These end portions may have welded thereto as at "24, 25, portions of a lead material designated 26, 26' which should also be of the same nickel-iron alloy as the reeds 11 and 1.2. Altemately, should it be desired, the end portions 22, 23 may be bent upwardly and should possibly terminate centrally above the housing as shown in FIG. 1A. If the lead material 26, 26 is used as illustrated in FIG. 1, it is preferable that this lead material be of a thin wire so that it does not conduct heat from the vicinity of the reed switch to the junction which will now be described.

At the top of the coil there is located nickel-iron copper junctions as at 30 and 30 (see also FIG. 2). These junctions are formed between the nickel-iron wires 26 and the interconnection material such as conwhich also could be of other materials, such as copper stated as being copper for the sake of illustration but plated with gold, tinned copper, pure gold, or pure silver wire and have been chosen as the material which will not create an additional thermal junction at the circuit into which the reed relay is to be inserted. It is preferable that the conductors 31,31 be of a thin wire to avoid conduction of heat from the outside environment. It should be understood that for the purposes of this invention this thermal junction is not dependent upon any one material of conducting wire that leads from the switch, but can be of any material that forms a thermal junction with the nickel-iron alloy that forms part of the reed switch assembly. To maintain the two junctions 30 and 30 at the same temperature, they are attached to a thermally conductive electrical insulator designated 32. The two junctions 30 and 30 are at tached to opposite sides of this insulator 32, and if the insulator is properly constructed, this can be done by simply soldering them to the opposite'sides.

A junction which is-alternate to FIG. 2 is shown in FIG. 2A where wires 26, 31 and 26, 31' are molded into a wafer 35 of thermally conductive electrical insulator. In FIG. 2B a further alternate is shown where the thermally conductive electrical insulator 328 has mounted thereon metal films 33B in spaced relation and the wires 26, 31 and 26', 31 are soldered to these metal filmsas at 34B thus placing the connections on the same side of the insulating body instead of on o posite sides as in FIG. 2. I

As an example of a suitable insulator, beryllium oxide has proven to be an excellent material, although other materials with the desired properties are available, such as aluminum oxide A1 0 and boron nitride, BN. A suitable configuration for the insulator 32 and particularly one that is made from beryllium oxide which has a resistivity of IO ohm-cm and a thermal conductivity approaching that of some metals, is to first coat opposite sides of the chip with a molybdenum manganese film and then coat that film with a nickel film and then place. a gold flash coating on the outside. If this is done and'the proper film is provided, which film is generally designated 33 or 3313, solder such as 34 or 348 may completely hold the junction to the insulator 32 as well as making thermal contact with the insulator 32, 32A, 32B. Other thermally conductive materials may be used to establish thermal contact of the junctions and the insulator 32, 32A, 32B. It has been found that if the insulator 32 is made from beryllium oxide with the film flashings mentioned above that optimum size considering the best combination of electrical and thermal properties is found with a chip that is 0.150 inches long, 0.080 inches high and 0.015 inches thick. Additionally, the solder which is used to secure the thermal junction to the insulator 32, is preferably of either the tin lead type or the cadmium tin type, the latter to be preferred when a higher performance is desired.

, In FIG. 1B reference'numeral with an A suffix have been used for.slightly varied partconfigurations. Here the entire capsule 13A and thermally conductive electrical insulator 32A is within the coil 20A. The inequalize; heat at either side thereof and in other respects the construction is equivalent to FIG. 1.

Referring to FIG. 3 of the drawings there. is illustrated a self-contained low thermal EMF switch which can be actuated by anytype of magnetic influence. This construction comprises a glass capsule 40 with a pair of reeds 41, 42 contained therein with the reeds extending outwardly of the capsule. The portions which extend outwardly of the capsule may either be bent back around into the center of the capsule so as to overlie a variety of forms of which any of the embeddment.

plastics would be suitable and if high performance of -the switch is desired, then the encapsulation material should be a thermally conductive dielectric, for example, a beryllium oxide filled epoxy.

Referring to FIG. 4 there is illustrated a form of the invention that becomes necessary if-one utilizes as the switch an asymmetrical switch in which the contacts are not centered in the capsule or are brought out of the opposite ends of the capsule, with two different types of leads such as a solid lead 50 at one end and a tubular lead 51 at the opposite end. Lead material 52, 52' of the same alloy as the reeds connect to an insulator 32 where the thermal junctions with the conductor material 54 are made. This type of switch must be displaced along the axis of the actuating coil 53 until a symmetrical position is found experimentally at which point the thermal EMF across the switch measured at conductors 54 is substantially zero. This is best achieved by having the coil energized so that the same is hot and then manually adjusting the position of the switch until the proper position is found and then securing the capsule-switch in position with epoxy or l. A reed relay comprising a coil having an elongated opening, a sealed magnetic switch having an elongated other thereby reducing the thermal EMF across the reed relay.

2. A reed relay as in claim 1 wherein lead material identical to the reeds is secured to the reed end portions extending from the housing and extend to the center of the housing exterior at which location the conductor members are secured to the leads.

3. A reed relay as in claim 1 wherein the thermally conductive electrical insulating body is within'the elongated opening in the coil.

4. A reed relay as in claim 1 wherein the thermally conductive electrical insulating body is outside the coil.

5. A reed relay as in'claim 2 wherein the thermally conductive electrical insulating body is oriented perpendicular to the coil axis. I

6. A magnetic reed switch comprising a capsule containing ferro magnetic reeds having portions extending therefrom, conductor members electrically connected to said portions and forming a juncture of dissimilar metals, a body of thermally conductive electrical insulating materiaL-each of the junctures being brought into-thermal contact with said body and so located as to be electrically insulated from each other whereby the thermally active junctions are maintained at substantially the same temperature and the thermal EMF s produced by the junctions being equal and opposite tend to cancel each other thereby reducing the thermal EMF across the reed switch.

7. A magnetic reed switch as in claim 6 wherein lead material identical to the reeds is secured to the reed end portions and extend to the center of the capsule. 

1. A reed relay comprising a coil having an elongated opening, a sealed magnetic switch having an elongated housing, ferro magnetic reeds mounted in said housing and having portions thereof extending therefrom, conductor members electrically connected to said reeds to form connections of dissimilar metals, a body of thermally conductive electrical insulating material, each of said connections being brought into thermal contact with said body and so located as to be electrically insulated from each other whereby the thermally active junctions are maintained at substantially the same temperature and the thermal EMF''s produced by the junctions being equal and opposite tend to cancel each other thereby reducing the thermal EMF across the reed relay.
 2. A reed relay as in claim 1 wherein lead material identical to the reeds is secured to the reed end portions extending from the housing and extend to the center of the housing exterior at which location the conductor members are secured to the leads.
 3. A reed relay as in claim 1 wherein the thermally conductive electrical insulating body is within the elongated opening in the coil.
 4. A reed relay as in claim 1 wherein the thermally conductive electrical insulating body is outside the coil.
 5. A reed relay as in claim 2 wherein the thermally conductive electrical insulating body is oriented perpendicular to the coil axis.
 6. A magnetic reed switch comprising a capsule containing ferro magnetic reeds having portions extending therefrom, conductor members electrically connected to said portions and forming a juncture of dissimilar metals, a body of thermally conductive electrical insulating material, each of the junctures being brought into thermal contact with said body and so located as to be electrically insulated from each other whereby the thermally active junctions are maintained at substantially the same temperature and the thermal EMF''s produced by the junctions being equal and opposite tend to cancel each other thereby reducing the thermal EMF across the reed switch.
 7. A magnetic reed switch as in claim 6 wherein lead material identical to the reeds is secured to the reed end portions and extend to the center of the capsule. 